Method and an arrangement for dampening vibrations in a mast structure

ABSTRACT

A method and an arrangement for dampening vibration in a mast structure of a mast truck where the critical characteristics of the mast structure and/or of the vibration are measured, computed, measured and computed or at least some of the characteristics are fed to the computing system in advance, the lowest natural frequency (ω n ) of the mast structure and/or the phase of the vibration is computed from the collected data or determined directly from the mast structure, the order for movement is obtained from driver or master system, the critical characteristics are fed to the computing system, the movement guidance to the speed controller is generated from above mentioned characteristics of the vibration, the order for movement is divided at least two parts (impulses I), and the actuator is controlled with speed controller for moving the truck or load in the truck according to the order for movement.

TECHNICAL FIELD

The invention relates to a method and an arrangement for dampeningvibration in a mast structure. More specifically the invention relatesto active dampening of vibration in mast trucks.

BACKROUND OF THE INVENTION

Mast trucks are used for lifting up persons or load. The mast truck hastwo main components: body of the truck and telescopic lifting device (amast). The mast is connected at it's lower part to the body of thetruck. In case when the load has to be moved for example to the shelfthe load must be moved to desired direction. There are threepossibilities to move load to the shelf. First choice is to drive thewhole truck itself forward. Second choice is to tilt the mast forward.Third choice is to use special device (a sledge) designed to reach outto the shelf. In all mentioned cases the load is moved back or forth inthe trucks driving direction. On the other hand the load can also bemoved sideways by changing the position of the forks or moving thesledge sideways.

When the truck is moved or the load is moved these movements areawakening vibration in the mast structure. This vibration becomes moreintense when the distance between the masts connection point and theload grows. This distance could easily be even over ten meters. Thisvibration is characterised by the masts structural characteristics, suchas mass and rigidity. These characteristics define the natural frequencyof the mast structure.

When the mass is moved (generation of the impulse) at the upper end ofthe truck mast the most critical vibration frequency is the lowestnatural frequency. When the mass is mounted to the mast and deflectedfrom its centre position and released the mass starts to vibrate at afrequency characterised by the mass and spring rate. The dampingcoefficient determines the rate how fast the vibration dies out.

The vibration in truck mast is most disturbing when the load is heavyand it is lifted to very high positions. This means that in this type ofcases the natural frequency is very low, typically around 0.3 Hz. Thistype of vibration is clearly visible and has large movement at the upperend of the mast and also this type of vibration is dies out very slowly,because the damping coefficient is affected only with the structures owndampening capabilities and air resistance.

The vibration in mast trucks is conventionally tried to be controlledwith structural options and solutions. Used techniques are for examplestiffening the mast structure, limiting the maximum speed of movement oravoiding rapid accelerations and deceleration.

On the other hand various passive systems have been developed to dampthe vibration of mast structures. The vibrating structure is in thosecases separated from the body of the truck with a damper or dampingmaterial. This type of solutions are operating with means of diminishingthe influence of the awakening impulse or by eliminating out thevibration energy from the structure. Another passive solution is also adynamic mass-damper, which vibrates at the opposing phase of vibrationthan the structure itself. The mass-damper is always adjusted to operatein certain frequency and when the natural frequency of the structure ischanged (different mass at the end of the truck mast) the mass-damperbecomes less effective. In worst scenario the mass-damper can become adevice which actually strengthens the amplitude of the vibration.

In documents EP 0 427 001 A1, DE 40 19 075 A1 and GB 2 379 434 A arepresented previously mentioned solutions.

In EP 0 427 001 A1 and DE 40 19 075 A1 are presented passive dampingsystems with separate dampers assembled between the truck mast and thetruck body. Systems are quite similar, only the positioning of thedamper system varies. In GB 2 379 434 A is presented a mass-dampersystem for controlling the vibration of the structure.

SUMMARY OF THE INVENTION

The object of the invention is to produce a method and an arrangementthat are effectively damping the vibration of mast trucks mast or evenpreventing the appearance of the vibration.

This object is achieved with an active damping arrangement havingfeatures described in the independent claim 1 and with a method havingfeatures described in independent claim 7.

The idea of the invention is to bring additional energy to the vibratingsystem at different phase of the vibration to even out the vibration oncontrary to the prior art systems that are trying to reduce the energywith dampers or such.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the exemplary embodiments of the invention aredescribed more specifically with reference to a drawing, wherein

FIGS. 1 a-b are showing different possibilities for the generation ofthe impulse,

FIG. 2 shows three lowest natural frequencies that can be generated inthe mast structure,

FIGS. 3 a-b are showing two prior art methods for passive systems,

FIG. 4 shows the impulse and counter-impulse in time/position scale, and

FIG. 5 shows the diagram of the method for dampening the vibrationaccording to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIGS. 1 a-b are described the different possibilities for thegeneration of the impulse (one degree of freedom). In FIG. 1 a arepresented four different movements that can generate vibration to themast structure 1 of the mast truck 2. All these movements are in themoving direction of the truck 2. Arrow 3 is describing the drivingmovement of the truck 2. The movement can be forward or backward. Arrow4 describes the movement of the mast structure 1 in relation to thetruck body 5. Also this movement can be either forward or backward.Arrow 6 is describing the tilting movement of the mast structure 1 inrelation to the truck body 5. The mast structure 1 can also be tiltedeither forward or backward. Arrow 7 is describing the movement of theforks 8 or a sledge that is carrying the load 9.

In FIG. 1 b are another possibility to generate vibration to thestructure. In this case the load 9 is moved in transversal directionaccording to an arrow 10 compared to the moving direction of the truck2. This movement is caused with moving forks 8 or a sledge.

In FIG. 2 are described three lowest natural frequencies (ω_(n)) thatcan be generated in the mast structure. Mass (load) is described as asquare 11 at the top of the mast 12. Also the mass of the mast 12 has tobe included to the calculations of the natural frequencies of thestructure. Another characteristics that is influencing to the naturalfrequencies is the stiffness of the mast structure (spring rate=k) andthe damping constant (c) (partly build in the structure and alsoincluding air drag). The most important frequency is the lowest naturalfrequency (described left).

In FIGS. 3 a-b are described two passive prior art methods forcontrolling the vibrations. In FIG. 3 a the mast structure 12 isisolated and the vibrations are damped with a damper 13. Load 11 issituated at the top of the mast 12. In FIG. 3 b is presentedschematically a dynamic mass damper system. The balancing mass (m) 14and the damper 14 are situated also to the top of the mast. Thissolution is fixed to certain mass (M) and to certain height of the mast12. In both figures M is a mass, c is a damping factor and k is a springfactor.

Also semi-active methods are known, where a specific characteristics ofthe system can be adjusted. Such characteristics are for exampleviscosity of the damper, rigidity of the joint or friction.

In FIG. 4 is presented active method for controlling vibration in astructure. X-axis presents time (seconds) and Y-axis presents position(centimetres). At time 0.0 seconds the structure has impact type impulseI₁, which activates the structure to vibrate according to curve 15. Theposition of the structures measuring point moves nearly 3 centimetres in0.2 seconds time and after that the measuring point starts to move backto the starting point (zero movement). After the half wavelengths time(here 0.4 s) the structure has reached again its original position andis having another impulse I₂ (a counter impulse). If the rate of theimpulse I₂ and the half wavelength is calculated correctly the secondimpulse I₂ stops the vibration of the structure. The dashed lines 16 and17 are demonstrating the movements generated by the impulses (I₁ and I₂)if they were affecting alone in the structure. In this example theaffecting time of the impulse is very short (hit type impulse).

In FIG. 5 is presented the diagram of the method and of the arrangementfor dampening the vibration according to the invention. The basic ideaof the invention is to divide the order of movement to at least twoimpulses that are affecting the mast structure. The first impulse causesthe mast structure to vibrate and the second (and possible other laterimpulses) are dampening the generated vibration. There are many possiblemathematical solutions to create such formulas that are counting themagnitudes of the impulses, the affecting time of the impulse and thetime gaps between the impulses. This calculation can be done so that atonce a part of the order of the movement is executed immediately whenthe order of the movement is received and another part after calculatedtime period. This guidance to the next moment of time and to the futureare calculated continuously. Another possibility is that the calculationcan be done as a continuous process where the affecting impulse iscalculated for every predetermined time period for next moment of timeand has feedback data of for example the movement of the tip of themast. The position of the tip of the mast can be calculated if the givenorder for movement and natural frequency are known or measured ormeasured and calculated by position sensor.

Both mentioned solutions are producing at least two impulses to the maststructure.

The method has following steps:

-   -   critical characteristics of the mast structure 12 and/or of the        vibration are measured, computed, measured and computed or at        least some of the characteristics are fed to the computing        system in advance,    -   the lowest natural frequency (ω_(n)) of the mast structure 12        and/or of the vibration is computed from the collected data or        determined directly from the mast structure,    -   the order for movement is obtained from driver or master system        (if automated storage with automated trucks),    -   the critical characteristics are fed to the computing system,    -   the movement guidance to the speed controller is generated from        above mentioned characteristics of the vibration,    -   the order for movement is divided at least two parts (impulses        I),    -   the actuator is controlled with speed controller for moving the        truck or load in the truck according to the order for movement.

The arrangement according to the invention has following features:

-   -   means for determining directly the natural frequency or the        phase of the vibration or means for determining the        characteristics for computing the natural frequency (ω_(n)) or        the phase of the vibration of the mast structure,    -   means for determining the order for the movement from the driver        or the master system,    -   means for computing the ratio of division and a time between the        impacts of the impulses and/or the phase of the vibration        generated by the order of the movement, and    -   means for computing the movement guidance to the speed        controller and delivering the guidance to the actuator.

The measuring of the critical characteristics of the mast structure canbe done with sensors that are measuring the amount of the load that islifted, the height of the load (length of the mast). Some of thesecharacteristics can be calculated from measured values and some of thesecharacteristics can also be fed in advance in the computing system, suchas mass distribution of the mast structure, rigidity of the mast, etc.

The measuring of the phase of the vibration directly from the structurecan be done by using different type of sensors such as strain gauges,acceleration sensors, speed sensors or position sensors. This embodimentis minimising the calculation process. The measurement of the phase ofthe vibration can also be done from the hydraulic system with pressuresensor.

Another possibility is to measure the amount of the mass (load), heightof the mass (lifting height) and then compute the phase of the vibrationbased on the information about the changes in the natural frequencies.

The active dampening of the vibration (lowest natural frequency of thestructure) in mast structure is done by modifying the given command formoving the truck or the load. In practice this means that given speedcommand is not carried out straight-forward, but so that the movement isdecelerated or accelerated in right phases of the vibration so that thevibration is damped.

One possible solution is to divide the impulse (acceleration) into twoparts and carry out the second (later) part of the impulse(acceleration) in phase of the reversing movement of the vibration andgenerate by this way a second impulse (counter impulse) to thestructure. By this arrangement the vibration dies out as is described inFIG. 4. The time between the two impulses is counted from the wavelengthof the natural frequency. The magnitude of the impulses (ratio ofdivision) and the time of execution and the affecting time of theimpulse are determined from the characteristics of the vibratingstructure and from the order for movement.

Another possibility is to divide the impulse (acceleration) intomultiple parts and by this way smoothen up the acceleration (moving ofthe truck or load) and affect the later parts of the vibration. With thelength of the time period can be affected to the smoothness of themovement, the longer the time period is the smoother the movement is.

Yet another possibility is to calculate new guiding parameters for everyfollowing predetermined time period. In the computer there is a formulafor the movement of the tip of the mast and for order for speed of themovement. To this formula can be entered for example the desired dampingfactor, such as 70%. (in undamped mast structures the damping factor isabout 2%) instead of trying to eliminate the whole vibration at once.Also the movement of the tip of the mast can be determined directly witha position sensor and be entered to the formula with desired dampingfactor.

In this case the instructions to the actuator are calculated for everyfollowing predetermined time period. The formula is generated so that itis generating counter impulse to the structure for dampening thevibration but on the other hand is directing the instructions to thesmoother direction, because one of the guiding factors is damping factor(70%).

The present invention may me embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respect only as illustrative andnot restrictive. The scope of the invention is, therefore, indicated bythe appending claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A method for dampening or preventing a vibration of a mast structureof a mast truck, wherein critical characteristics of the mast structureand/or of the vibration are measured, computed, measured and computed oroptionally some of the characteristics are fed to a computing system inadvance, the lowest natural frequency (ω_(n)) of the mast structureand/or the phase of the vibration is computed from the collected data ordetermined directly from the mast structure, the order for movement isobtained from a driver or master system, the critical characteristicsare fed to the computing system, the movement guidance to a speedcontroller is generated from above mentioned characteristics of thevibration, the order for movement is divided at least into two parts(impulses I), and an actuator is controlled with the speed controllerfor moving the truck or load in the truck according to the order formovement.
 2. The method according to claim 1, wherein the ratio ofdivision of the impulses, the affecting time of the impulses, the timebetween the impacts of the impulses are determined from the computed ormeasured natural frequency (ω_(n)) and/or phase of the vibration andfrom the order of the movement.
 3. The method according to claim 1,wherein the calculation of the impulses and the guidance to the speedcontroller is a continuous process, where the affecting impulse iscalculated as a continuous process from the order of the movement, fromthe natural frequency (ω_(n)) and/or from the phase of the vibration forevery predetermined time period.
 4. The method according to claim 1,wherein the critical characteristics of the mast structure are measuredwith strain gauge(s), acceleration sensor(s), speed sensor(s) and/orposition sensor(s).
 5. The method according to claim 1, wherein thecritical characteristics of the mast structure are measured from ahydraulic system of the mast truck with a pressure sensor(s).
 6. Themethod according to claim 1, wherein the phase of the vibration of themast structure is obtained by calculating it from the height of theload, magnitude of the load and changes caused by the load in thenatural frequency.
 7. The method according to claim 1, wherein the phaseof the vibration of the mast structure is obtained directly from thestructure by using strain gauge(s), acceleration sensor(s), speedsensor(s) and/or position sensor(s).
 8. The method according to claim 1,wherein the natural frequency (ω_(n)) of the mast structure iscalculated from the height of the load, magnitude of the load andchanges caused by the load in the natural frequency.
 9. The methodaccording to claim 1, wherein the natural frequency (ω_(n)) of the maststructure is obtained by measuring the vibration by using straingauge(s), acceleration sensor(s), speed sensor(s) and/or positionsensor(s).
 10. The method according to claim 3, wherein the guidance tothe speed controller is computed continuously for every predeterminedtime period and affected also with a desired damping factor.
 11. Anarrangement for dampening or preventing a vibration of a mast structureof a mast truck, wherein the arrangement comprises: means fordetermining directly the natural frequency (ω_(n)) or the phase of thevibration or means for determining the characteristics for computing thenatural frequency (ω_(n)) or the phase of the vibration of the maststructure, means for determining the order for a movement from thedriver or the master system means for computing value of the order formovement at different phases of the vibration, and means for computingthe movement guidance to a speed controller and delivering the guidanceto the actuator for moving the truck according to the order formovement.
 12. The arrangement according to claim 11, wherein the meansfor determining the characteristics of vibration of the mast structureare strain gauge(s), acceleration sensor(s), speed sensor(s), positionsensor(s) and/or pressure sensor(s).
 13. A mast truck having anarrangement for dampening the vibration of the mast structure accordingto claim
 11. 14. A mast truck having an arrangement for dampening thevibration of the mast structure according to claim 12.